This is a continuation of a series of posts on the Nikon D850. You should be able to find all the posts about that camera in the Category List on the right sidebar, below the Articles widget. There’s a drop-down menu there that you can use to get to all the posts in this series; just look for “D850”.
I’ve had two people tell me that the right AF Adjustment values for the D850 vary with the color temperature of the lighting. The purpose of this post is to find out if that’s true. But first, I’m going to walk you through how the D850 autofocus works with a new (to this series) lens: the Nikon 200 mm f/2G VR I.
First, let’s look at focus shift with aperture when the lens is focused manually at a constant distance, which happens to be 5.3 meters:
I’ve plotted the three Adobe RGB color channels. The graph presents displacement of the image projected on the sensor from the desired green-channel focal plane. Negative numbers indicate front-focusing. The image-plane shift is in micrometers (um). The separation of the focal distances of the three color planes is because of the longitudinal chromatic aberration (LoCA) of the lens. The dots indicate the results for each of the ten exposures at each f-stop. I’ve plotted lines indicating the average (aka mean or mu) of the sample set bolder and added thin lines above and below the means that are one standard deviation (sigma) away from them.
This lens has very low LoCA and pretty low focus shift.
Let’s look at the circles of confusion (CoCs) implied by the above graphs:
All the errors are within one pixel pitch, and in many cases well within that.
Now we’ll look at the errors with autofocus set to spot mode, using AF-S and the Westcott 1×2 foot panel that was the illuminant set to 6000 degrees Kelvin:
Those are really impressive. Plus one sigma to minus one sigma is within two pixel pitches. Looks like +2 on the AF adjustment setting would make it even better.
Now with the lights set to 3000 degrees Kelvin:
Lowering the color temperature of the lights dramatically makes the lens focus the image on the sensor about 4 micrometers in the front-focusing direction. This is a very small change; about one AF adjustment setting.
Cranking in that +2 AF adjustment with the 6000K illuminant:
Nothing to complain about here.